The present invention relates to a linear motor in accordance with the embodiments disclosed herein.
A linear motor typically comprises a stator, a linearly movable armature as well as drive and control electronics. In order to move the armature, as a rule, either the stator or the armature is provided with excitable windings, i.e. drive windings, and, correspondingly, either the armature or the stator is provided with permanent magnetic excitation. The permanent magnetic excitation is most commonly generated by means of permanent magnets arranged in a certain manner. Whether the permanent magnets are disposed in the stator or in the armature and, accordingly, whether the excitable windings are disposed in the armature or in the stator, are determined by the desired field of use and/or the conditions there.
Linear motors may be classified into linear motors having the armature arranged on the inside, a so-called internal armature, and linear motors having the armature arranged on the outside, a so-called external armature.
In the case of a linear motor having an internal armature, the armature essentially comprises permanent magnets which generate a magnetic field with alternating polarity. A conventional embodiment of such an armature consists of a thin metal tube, e.g. of chrome steel, having a wall thickness of several tenths of a millimeter, into which disk-shaped, axially polarized neodymium magnets have been pressed, between which, if necessary, spacers made of magnetic or non-magnetic materials are placed.
The stator of a linear motor having an internal armature comprises, as a rule, a winding former which has the dual function of both carrying the drive winding and serving as a slide bearing for the armature. The distance between the armature and the winding former is kept as small as possible, typically at 1/10 mm (constructional air gap). The winding former has a wall thickness in the area of the slide bearing, which is selected as thin as possible to ensure that the gap (basically an air gap) between the drive winding and the armature does not become excessively large and, as a result, the magnetic force generated by the drive winding is not unnecessarily reduced. The wall thickness in the area of the slide bearing is typically less than one millimeter and is first of all dependent on the size of the linear motor. It is also conceivable to design the drive winding out of several winding packages, so-called air-cored coils, which are arranged together in rows and are subsequently encapsulated or bonded together, so that a protective plastic layer is formed around the drive winding. However, for manufacturing reasons, a discrete winding former is used in most cases, onto which the windings are wound or pushed as packages.
In most applications, the slide bearing has a service life which exceeds by far that of the linear motor itself. However, what may be disadvantageous are transverse forces acting on the armature, which may, depending on the wall thickness of the winding former, result in wear of the slide bearing. This may lead to the armature penetrating through the winding former in the area of the slide bearing. Such penetration will on the one hand cause a total failure of the linear motor. However, what may be even more serious is the safety problem due to the risk of the armature coming into contact with the drive winding. Although the drive winding has insulated winding wires, typically including a varnish insulating layer, this layer has a thickness of only several hundredths of a millimeter. Thus, the varnish insulating layer may be abraded within a very short period of time and the armature will subsequently be in contact with the live drive winding, which is to be avoided in any case. Since this process proceeds at a very slow rate, the armature may at times already have the potential of the drive winding, although the linear motor is still operable. This constitutes a serious safety problem, since the drive winding of the linear motor needs to be dimensioned for large loads having high currents and high voltages, with such high currents and high voltages bearing a risk to human lives. According to the prior art it is suggested that the armature be earthed. This is, however, laborious, complex to manufacture and requires a great deal of maintenance during operation. Other possibilities include a separate voltage measurement on the armature—which is also complex—or the provision of a separate guiding mechanism for the armature externally of the winding former, which leads to an unnecessary increase in the dimensions of the linear motor and also limits the maximum possible stroke of the armature.